Air strikes by the U.S.-led coalition killed at least 21 civilians in and around a stronghold of the Islamic State (IS) extremist group in northern Syria on July 18, according to a London-based monitoring group.
The Syrian Observatory for Human Rights said that at least 15 civilians were killed in a district of Manbij while six others were killed in a village near the city.
Kurdish-led Democratic Forces of Syria (DFS) launched a campaign to take Manbij from IS militants at the end of May.
The operation is intended to cut IS militants off from their last access route to the Turkish border and prevent fighters and supplies from reaching the group's de facto Syrian capital of Raqqa.
The operation has caused thousands of civilians to flee but tens of thousands of them are still trapped in Manbij.
The DFS has seized territory from IS across large parts of northern and northeastern Syria.
Capturing Manbij would be its most significant victory yet.